Levansucrase (sucrose; 2,6-.beta.-D-fructan 6 .beta.-D-fructosyltransferase E.C.2.4.1.10) is an extracellular enzyme secreted by Bacillus subtilis after induction which occurs following addition of sucrose to the growth medium. Sucrose induces expression of a sacB gene, which encodes levansucrase. Sucrose also induces expression of at least two other structural genes, sacA, which codes for an endocellular sucrose-6-phosphate hydrolase commonly referred to as sucrase, and sacP, which codes for a membrane component of the phosphotransferase system of B. subtilis which is involved in sucrose uptake by cells. Genetic and biochemical data concerning uptake and metabolism of sucrose support the view that at least eight loci (sacA, sacB, sacP, sacQ, sacR, sacS, sacT, and sacU) are involved in specific and pleiotropic regulatory mechanisms in the sucrose system of Bacillus subtilis 168 [reviewed by Lepesant et al. in Schlessinger, D (Ed.) Microbiology-1976, American Society for Microbiology, Washington, D.C., 1976, pp. 58-69]. Of theses loci, nucleotide sequences have been described for sacA (Fouet et al., Gene 45:221-225, 1986), sacB and sacR (Steinmetz et al., Mol. Gen. Genet. 200:220-228, 1985). The positive regulator gene of the sacS locus is involved in inducible expression of sacB, the gene for levansucrase. The sacB structural gene is preceded by a regulatory region referred to as sacR. The sacR regulator region consists of a constitutive promoter which is followed by a stem-loop region which acts as a rho-independent transcription terminator in the absence of sucrose [Aymerich et al., J. Bacteriol. 166; 993-998, 1986; Shimotsu et al., J. Bacteriol. 168; 380-388, 1986; Zukowski et al., Gene 46:247-255, 1986]. Thus transcription from the constitutive promoter in the sacR regulatory region is arrested by the transcription terminator in the absence of sucrose, mRNA for the sacB structural gene is not synthesized, and levansucrase is not produced. Therefore, in order for sucrose induction to occur, the transcription terminator must be rendered nonfunctional by an anti-terminator.
Regulation of gene expression has been comparatively less characterized in gram-positive bacteria, such as Bacillus subtilis, than in the gram-negative entero-bacteria, such as Escherichia coli. Regulation of gene expression at the level of transcription has been successfully employed in developing several gene expression vectors for efficient, high-level production of foreign proteins in E. coli. However, for B. subtilis host/vector systems, only two regulated systems have been developed. One system uses E. coli lac operator-repressor in B. subtilis, resulting in an IPTG-inducible expression of heterologous genes [Yansura & Henner, Proc. Natl. Acad. Sci. U.S.A. 81:439-443, 1984]. This system has limited applications due to the expense associated with the IPTG inducer. The second system is based on thermo-inducible gene expression in B. subtilis utilizing transcriptional regulatory elements from the temperate bacteriophage phi-105 [Dhaise et al., Gene 32:181-194, 1984]. This system requires two plasmids in the same host cells, thus increasing the difficulty involved in stably maintaining the host cells and also requires elevated temperatures (.about.45.degree. C.) for induction, wherein such elevated temperatures may be detrimental to the efficient production of proteins which are sensitive to heat.